Bulk CuInS2 has a direct band gap of 1.53 eV, and Bulk Cu2ZnSnS4 has a direct band gap of 1.45-1.51 eV. These each correspond well with the solar spectral range needed for photovoltaic applications. The band gap of bulk Cu2ZnSnS4 is larger than the 1.05 eV of CuInSe2 and similar to the 1.53 eV band gap of CuInS2.
It has been reported that the theoretical efficiency of 28.5% of CuInS2 solar cells is the highest among the Cu chalcopyrite solar cells (S. Siebentritt, Thin Solid Films, 403-404 (2002) 1). CuInS2 solar cells are expected to show efficiencies superior to those of Cu(In,Ga)Se solar cells. It has been reported that the theoretical efficiency of ˜30% of Cu2ZnSnS4 solar cells is one of the highest among the quaternary chalcogenides solar cells (S. Botti et al., Applied Physics Letters, 98, 241915-8 (2011)).
To date, the efficiency of solar cells based on sulfide has reached only about 60% of the selenides resulting in continuing efforts to improve the efficiency of CuInS2 solar cells. Solar cells based on a CuInS2 absorber layer prepared by vacuum deposition techniques have reached efficiencies of 11.4% (S. Siebentritt, Thin Solid Films, 403-404 (2002) 1). Cu2ZnSnS4 solar cells are expected to show efficiencies superior to those of Cu(In,Ga)Se solar cells. Solar cells based on a Cu2ZnSn(S,Se)4 absorber layer prepared by vacuum deposition techniques have reached efficiencies of 9.7% (T. Todorov et al., Advanced Materials, 22, E156-9 (2010)). Other low-cost thin film deposition methods have been employed in an effort to reduce production costs, such as electrodeposition (S. Pawar et al., Electrochimica Acta, 55, 4057-4061 (2010); S. Nakamura and A. Yamamoto, Solar Energy Materials and Solar Cells, 1997, 49, 415-421), and spray deposition (Y. Kishore et al., Solar Energy Materials and Solar Cells, 93, 1230-7 (2009); (Albert Goossens and Joris, H. Nanotechnology, 19 (2008) 424018).
Recently, photovoltaics based on solution-processed nanoparticles have attracted attention, such nanoparticles having advantageous handling characteristics. Li et al. fabricated CuInS2 solar cells using an in situ nanoparticle synthetic method and obtained an efficiency of up to about 4% (Li, L.; Coates, N.; Moses, D. J. Am. Chem. Soc. 2009, 132, 22). Nanoparticle ink has the advantage of mono-dispersion in organic solvents for coating, but almost all reported nanoparticles have been found to need rebuilding crystallinity and stoichiometry via annihilation, a process that needs to be carried out at a relatively high temperature, often as high as 600° C., and in a controlled environment containing ambient sulfur and selenium. The encapsulating organic ligands used for stable dispersion of nanoparticles are “burned out” by the annihilation process leaving unwanted residues that lower the particles' photovoltaic L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. J. Am. Chem. Soc. 2008, 130, 16770.). In addition to all this, current methods for producing CuInS2 and Cu2ZnSnS4 nanocrystals are usually air-sensitive and generally carried out using a Schlenk-line or equivalent apparatus.